Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator rotor may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology.
DFIG operation is typically continuously pushed toward a wider speed range to increase annual energy production (AEP). In certain instances, however, it has been observed that under over-speed operating conditions, the rotor modulation index is constantly around or even higher than 1.0. Thus, whenever there is a grid event such as a high-voltage ride through (HVRT) or sub-synchronous resonance, this narrow margin (between the steady state modulation index and a higher modulation index such as 1.15 and beyond) can quickly disappear and the DFIG may lose its stable control.
To address this issue, certain DFIG systems include a reactive current logic under high-slip conditions so as to reduce the rotor voltage by commanding inductive current whenever a high level of stator voltage is expected. This logic takes the stator voltage magnitude as the major input and uses high pass filter and proportional gain to generate an inductive current command to reduce rotor voltage. Though this logic has been proven effective, over-modulation at the rotor converter is observed in a few cases which fails to assists the DFIG in regaining stability.
In view of the aforementioned issues, a control methodology that calculates the maximum reactive current given the maximum rotor modulation index and one or more DFIG operating conditions rather than using the stator voltage magnitude and a regulator-type of control to limit reactive current would be advantageous.